Project Summary/Abstract Kidney transplantation is the treatment of choice for patients with end-stage renal disease. Although short-term outcomes are excellent, long-term allograft survival rates remain inadequate. It has been increasingly recognized that anti-HLA donor-specific antibodies (DSA) play a significant role in the development of chronic rejection and fibrosis that lead to late renal allograft loss. The mechanisms that underlie the generation of DSA are poorly understood; therefore, a better understanding of the cellular mechanisms responsible for DSA formation following transplantation has potential to guide the optimization of current immunosuppressive strategies and the development of novel therapies to control DSA. The applicant for this K08 career development award is an early career abdominal organ transplant surgeon with a strong background in preclinical nonhuman primate models and clinical transplantation seeking to enhance his knowledge of basic immunology and acquire advanced skills in experimental mouse models to answer the fundamental mechanistic questions necessary to realize bench-to-bedside translation of novel approaches to control DSA in kidney transplant recipients. In this application, he proposes to examine the costimulatory and coinhibitory mechanisms that drive T follicular helper (Tfh) cell-mediated DSA responses in the setting of CD28 costimulation blockade in a murine skin transplant model. Through a series of interrelated experiments, he will test the hypothesis that improved inhibition of Tfh cell and DSA responses following transplantation with selective CD28 blockade is dependent upon the coinhibitor CTLA-4, and that CTLA-4 is mediating its coinhibitory effect on DSA in a Tfh-intrinsic manner. The work will also examine if reduced expression of the costimulator ICOS on Tfh cells is a mechanism of anti-CD28-induced DSA inhibition. The overall goal of this project is to elucidate the mechanistic underpinnings of costimulation blockade-mediated Tfh cell inhibition and DSA prevention. The proposed work is aligned with the NIAID strategic priority of identifying therapeutic targets for the enhancement of translational efforts to extend renal allograft survival through preclinical research. A first-class mentor team, led by primary mentor Mandy Ford, PhD, and co-mentors Christian Larsen, MD, DPhil and Larry Boise, PhD, will support the applicant's immediate goal of launching his independent research career. In the process, through formal coursework and a structured professional development plan, this K08 will help the applicant build a platform from which to pursue his long-term career goal of creating a translational research program that links findings on the basic mechanisms of Tfh cell-mediated DSA formation to clinical observations in transplant recipients to advance the ability to control anti-HLA antibodies in kidney transplantation. !